1. Field of the Invention
The present invention relates generally to optical fiber systems and particularly to suppression of second order distortions using dispersion compensation.
2. Technical Background
Optical fiber transmission systems were developed some thirty years ago for long-distance telecommunication because of their immunity from electromagnetic interference, large bandwidth, light weight, and other advantageous properties. The typical optical fiber has a high-refractive-index core region surrounded by a low-refractive-index cladding. A protective coating is usually provided over the cladding to protect the structure from the environment.
The distance over which optical signals can be transmitted over an optical fiber is limited by attenuation due to absorption and scattering (e.g., Rayleigh, Brillouin and weak scattering), as well as from geometric effects (e.g., bending). Consequently, over the years tremendous effort has been directed to studying and reducing these sources of signal attenuation (loss).
Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty in many optical transmission systems. In many transmission systems, for example in networks carrying cable TV (CATV) transmission signals, it is desirable to transmit large optical power through optical fibers, while maintaining high signal to noise ratio (SNR). However, as the power of the incident optical signal launched into an optical fiber increases, it may exceed a certain threshold power (SBS threshold) and part of the signal power will then be reflected back due to SBS. Thus, due to SBS, a large amount of the signal power can be lost due to reflection back toward the transmitter. In addition, the scattering process increases the noise level at the signal wavelength. The combination of decrease in signal power and increase in the noise lowers SNR and leads to performance degradation.
An intense optical field (associated with the high power optical signal propagating through transmission fiber) generates pressure or sound waves through electrostriction due to the beating of intense incident and spontaneous reflected light, giving rise to pressure or acoustic waves. The change in pressure causes material density to change, thereby resulting in refractive index fluctuations. The net result is that an intense electrical field component of the optical wave generates pressure or sound (acoustic) waves which cause material density fluctuations. The acoustic wave changes the refractive index and enhances the reflected light amplitude through Bragg diffraction, thus resulting in SBS. Above the SBS threshold of an optical fiber, the number of stimulated photons is very high, resulting in a strong reflected field which limits the optical power that is transmitted, and which reduces the signal to noise ratio SNR.
Some approaches to solving this problem utilize phase modulation, which increases the SBS threshold power. However, the interaction of the phase modulation with the fiber dispersion leads to an increased composite second order CSO distortion. CSO distortion is undesirable because it degrades signal quality.